The following background information is provided to assist the reader to understand the environment in which invention will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.
A typical freight train includes one or more locomotives, a plurality of railcars and a pneumatic trainline referred to as the brake pipe. The brake pipe consists of a series of individual pipe lengths interconnected to each other. One pipe length secured to the underside of each railcar interconnects to another such pipe length via a flexible coupler situated between each railcar. The brake pipe supplies the pressurized air that is required by the brake control system to charge the various reservoirs and operate the air brake equipment on each railcar in the freight train.
A train operator situated in the lead locomotive can manipulate a brake handle to apply and release the brakes on the railcars as desired. The brake handle can be moved from and in between a release position at one extreme in which brake pipe pressure is maximum and the brakes are completely released to an emergency application position at another extreme in which brake pipe pressure is essentially zero and the brakes are fully applied. The brake handle positions thus include brake release, minimum service brake application, full service brake application and emergency brake application. When the brakes are released, the reservoirs and the brake pipe are generally charged to the same pressure: typically 90 psi on a freight train and 110 psi on a passenger train. When the brakes are applied, the pressure in the brake pipe is reduced typically through a valve located in the lead locomotive. The exact amount by which the pressure is reduced depends into which of the application positions the brake handle is placed. It is this pressure reduction that signals the brake control valve on each railcar to supply pressurized air from the appropriate reservoir(s) to the brake cylinders. The brake cylinders convert this air pressure to mechanical force and mechanical linkage transmits the mechanical force from the brake cylinders to the brake shoes. The brake shoes apply this mechanical force to slow or stop the rotation of the wheels on the railcar. Assuming the brake signal is successfully communicated throughout the train, the brakes of every railcar in the train respond in the generally same manner.
The brake equipment on each railcar of a freight train typically includes one or more brake cylinders, an emergency air reservoir, an auxiliary air reservoir and a conventional pneumatic brake control valve such as an ABDX, ABDW, DB60 or similar type control valve. The ABDX and ABDW brake control valves are made by the Westinghouse Air Brake Company (WABCO) and are well known in the brake control art.
FIG. 1 illustrates a schematic diagram of a pneumatic brake control system of a railcar featuring an ABDX type pneumatic brake control valve. This control valve includes a service portion and an emergency portion typically mounted on opposite sides of a pipe bracket. The pipe bracket features a number of internal passages and several ports. Each port connects to one of the interconnecting pipes from the railcar such as those leading to the brake pipe, the brake cylinder, the emergency reservoir, the auxiliary reservoir and the retaining valve. It is through these ports and internal passages of the pipe bracket that the relevant portions of the control valve communicate fluidly with the pneumatic piping on the railcar.
Railcars are also often equipped with a combination access and receiver assembly that enables an Automated Single Car Tester to measure the pressure at various points within the brake control system of the railcar. An access plate portion of the assembly is typically connected between the pipe bracket and the service portion of the brake control valve. A receiver portion together with the access plate provides access to the internal passageways of the pipe bracket. The combined assembly is the part through which the Automated Single Car Tester can measure pressure within the brake cylinder, the brake pipe, the emergency reservoir and the auxiliary reservoir on the railcar. The combined assembly and the Automated Single Car Tester are also both made by WABCO and well known in the brake control art.
The service and emergency portions of the pneumatic brake control valve operate according to principles known in the railroad industry. The service portion of the control valve performs several functions including (1) controlling the flow of air from the auxiliary reservoir to the brake cylinders during a service brake application, (2) controlling the recharging of the auxiliary and emergency reservoirs, and (3) controlling the exhausting of the brake cylinders when the brakes are released. The emergency portion of the control valve controls, among other things, the flow of air from both reservoirs to the brake cylinders during an emergency brake application. The emergency portion can also accelerate this increase in brake cylinder pressure by venting the brake pipe locally at the railcar.
The ABDX, ABDW, DB60 and similar type brake control valves respond to decreases in brake pipe pressure differently from how they respond to increases in brake pipe pressure. By way of example, a typical freight railcar may feature a brake control valve designed to work with a brake pipe that is chargeable to 90 psi. With the brake handle in the full release position, the brake pipe charges to 90 psi at which level the brake control valve completely depressurizes the brake cylinders thereby fully releasing the brakes on the railcar. By moving the brake handle back towards the application extreme of its spectrum, the brake pipe pressure would decrease accordingly. The brake control valve would respond to this decrease in brake pipe pressure by increasing pressure in the brake cylinders to apply the brakes. Should the brake handle be moved even further to the full service application position, the brake pipe pressure would decrease to approximately 68-70 psi. The brake control valve would respond by increasing the brake cylinder pressure to approximately 64 psi to apply the brakes fully. The brake control valve is thus capable of incrementally increasing pressure in the brake cylinders as the pressure within the brake pipe is decreased incrementally.
One disadvantage to such prior art brake control valves is that they are incapable of incrementally reducing the brake cylinder pressure in response to increasing brake pipe pressure. This is perhaps best illustrated by considering how a train is operated over hilly terrain. As a train descends a hill, the train operator may be required to apply the brakes to slow the train. By moving the brake handle towards the application extreme of its spectrum, the brake pipe pressure decreases and the brake control valve responds by allowing air from the auxiliary reservoir to flow into the brake cylinders to apply the brakes. Should the train slow too much, however, the train operator may need to increase speed again, for example, to maintain a schedule or to prepare for an upcoming hill.
There are two ways in which the speed of the train can be increased while descending a hill. Specifically, the train operator can either (i) modify the braking effort so that the train again reaches the desired speed or (ii) keep the brakes applied and increase the speed by engaging the propulsion motors of the locomotive. Regarding the former alternative, the ABDX, ABDW, DB60 and similar type brake control valves, however, do not allow the brake cylinders to be depressurized incrementally. Moving the brake handle back towards the release extreme of its spectrum does cause the brake pipe pressure to increase accordingly. But once brake pipe pressure increases beyond a set threshold (i.e., typically 2-4 psi above auxiliary reservoir pressure), the brake control valve completely empties the brake cylinders thereby releasing the brakes fully. For the train operator to slow the train using this approach, the only way to reduce braking effort from, say, a full service 64 psi to 20-30 psi brake cylinder pressure is to empty the brake cylinders completely and then repressurize them to the desired level. This is, of course, impractical as it takes minutes to do so, the exact time depending on the length of the train. Consequently, the only safe way to increase speed in such circumstances is to leave the brakes on and engage the locomotive engines to overcome the brake drag. This approach is quite inefficient as it, for example, increases both fuel consumption and wear of the brake shoes.